1. Field of the Invention
The present invention relates to a method for controlling the defrosting of a plurality of refrigerator-freezer units of varying degrees of frost accumulation cooled by a single compressor/condenser.
2. Description of the Prior Art
A cooling system wherein a plurality of refrigerator-freezer units of varying degrees of frost accumulation are cooled by use of a single compressor/condenser is known. The refrigerator-freezer units, for example, may comprise showcases for display of refrigerated or frozen foods. The respective showcases have respective evaporators for cooling the showcases. The respective evaporators are connected to a single condenser and a single compressor via respective expansion valves and solenoid valves. Defrosting heaters for defrosting a frost accumulated on the evaporators are provided on respective evaporators. When the evaporators are defrosted, all of the solenoid valves are closed, the drive of the compressor is stopped and electric current is supplied to all of the defrosting heaters to defrost the frost on the evaporators
The conventional control method for defrosting the evaporators is shown in FIG. 21. FIG. 21 illustrates a time chart in the case where three showcases are cooled by a single compressor. The time between periods of defrosting and the period of time for defrosting of the evaporators of the showcases are preset by a defrosting timer, for example, the time for beginning the defrosting is preset to an eight hour interval and the period of time for defrosting is preset to twenty minutes. When the defrosting timer switches operation to a defrosting mode, all of the solenoid valves are closed and the drive of the compressor is stopped which stops the cooling of the evaporators. At the same time, electric current is supplied to all of the defrosting heaters Ha, Hb and Hc, and the heaters warm the evaporators to dissolve and eliminate frost accumulated on the evaporators. The period of time of supplying electric current is the same (for example, twenty minutes) for all of the defrosting heaters as predetermined by the defrosting timer operation. This period of time is usually set to a sufficient time for defrosting that evaporator on which the amount of accumulated frost is the largest. Then, when the defrosting timer returns operation from the defrosting mode to its cooling mode, the compressor is again driven, all of the solenoid valves are opened and the cooling medium is circulated to all of the evaporators, thereby beginning the cooling of the showcases
In such a conventional control method, since electric current is supplied to the defrosting heaters while the compressor is stopped, the power source for the heaters and the power source for the compressor can be shared by switching the common power source from the compressor to the heaters. Therefore, the total capacity of the power source required for the entire showcase apparatus can be small in comparison with a situation where independent power sources are provided for the heaters and the compressor.
In the conventional defrosting control method, such as the one described above, however, since the period of time for supplying electric current to the defrosting heaters is set for that evaporator which requires the longest time for defrosting, the temperatures a and b inside of the showcases which have different evaporators increases to a level well above temperature c inside of the showcase having the evaporator requiring the longest defrosting period and becomes much higher than a temperature level necessary for completing the defrosting of the evaporators, as shown in FIG. 21. Accordingly, the goods contained in the showcases are unneccessarily warmed, thereby reducing the quality of any goods stored in those showcases. Moreover, the rise of the temperatures inside of the showcases causes the cooling loads of the showcases to increase when the cooling of the showcases begins after the defrosting, thereby greatly decreasing the cooling efficiencies of the showcases.
It is possible to shorten the period for defrosting in order to avoid these problems. However, if the defrosting time is shortened, some accumulated frost may remain on, for example, evaporator c. The remaining frost obstructs the cooling action of those evaporators, and any goods contained in the showcases may not be sufficiently cooled.
On the other hand, another system of showcases is known which has outer fans for forming air flow curtains which prevent outside air from entering the showcases. Alternatively, outside air may be conducted to the positions of evaporators when the drive of the outer fans stops. In this system as shown in FIG. 22, the drive of all of the outer fans Fa, Fb and Fc is stopped at the same time as electric current supply to the defrosting heaters Ha, Hb and Hc when a defrosting timer switches operation to its defrosting mode. Outside air is led to the positions of the evaporators by stopping the outer fans, the temperature of the air defrosts the evaporators, and the time for defrosting can be shortened in comparison with the situation where only the heaters are used during the defrosting mode.
Also in this control method, however, since the period of time for supplying electric current to the defrosting heaters is set for that evaporator which requires the longest time for defrosting, the temperatures a and b inside of the showcases which have different evaporators increases to a level well above temperature c inside of the showcase with the longest defrosting period. Therefore, the same problems as those aforementioned still remain.